1. Field of the Invention
This invention relates to illumination optics and, more particularly, to illumination pupil and field uniformity correction.
2. Description of the Related Art
The following descriptions and examples are given as background only.
Fabricating semiconductor devices such as logic and memory devices typically includes processing a substrate using a large number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during the manufacturing process to detect defects on wafers, promoting higher yield in the manufacturing process, and thus, higher profits. Inspection has always played an important role in the fabrication of semiconductor devices. However, the performance requirements of inspection systems has increased over the years, as a result of continually decreasing dimensions of the semiconductor devices. In particular, inspection systems require significantly higher resolution and sensitivity for detecting the small sized defects, which occur on advanced semiconductor wafers.
One factor affecting the resolution and sensitivity of the inspection system is the quality of light used to illuminate the specimen or semiconductor wafer. There are generally two types of light sources used within inspection systems. In some cases, a laser light source may be used to generate relatively bright light at relatively shorter wavelengths. However, laser light sources generate coherent light, which is undesirable for inspection for many reasons. For example, coherent light may produce speckle and/or ringing in the inspection images generated by the imaging sensor. Speckle decreases the resolution of the inspection system by decreasing the signal-to-noise ratio of the output signals generated by the system. Ringing introduces artifacts into the inspection images, which reduce sensitivity and make it difficult to detect defects. Although some illumination systems have been designed to reduce the speckle and ringing produced by coherent light sources, other systems avoid these problems altogether by illuminating the specimen with incoherent light.
An illumination system 100 comprising an incoherent light source is shown in FIG. 1. In an ideal case, the illumination system would produce a spatially uniform beam of light that covers the entire field of view (FOV) of the inspection system. The light generated by the illumination system would also have a uniform angular distribution over the entire numerical aperture (NA) transmission window of the inspection system. Any deviation from spatial or angular distribution uniformity will adversely affect the resolution and sensitivity of the inspection system, and therefore, is undesirable.
In the illustrated embodiment, illumination system 100 comprises a plasma arc lamp 110, an elliptical reflector 120, a homogenizer or lightpipe 150, and a pupil lens 160. The plasma arc lamp 110 may include any arc lamp which generates light in all directions and is suitable for inspecting a specimen. The elliptical reflector 120 is used to collect and reflect the light from arc lamp 110 to the entrance of homogenizer 150. The entrance of homogenizer 150 is located one focal length of the elliptical reflector away from the arc lamp.
In some cases, folding mirror 130 may be included so that some wavelengths of light (such as ultra-violet, UV, and deep ultra-violet, DUV wavelengths) are reflected to homogenizer 150, while other wavelengths (such as visible and infrared wavelengths) are transmitted through mirror 130 out of the optical path of the illumination system. Folding mirror 130 provides many advantages. For example, folding mirror reduces the system heat load, prolongs the lifetime of the illumination optics and reduces the noise background by removing unwanted visible and infrared light from the optical path. By folding the optical path (e.g., by 90°), folding mirror 130 reduces the system footprint and improves the rigidity and serviceability of the illumination system design.
Because the light generated by arc lamp 110 is typically not as bright as laser light, it is generally desirable that the elliptical reflector be configured to collect as much light as possible. The depth of the elliptical reflector is one factor that affects the intensity of the light generated at the illumination plane 140 of the illumination system. The reflector depth also affects the angular distribution of the generated light. For example, a deep ellipse may be used to provide greater intensity at the expense of a non-uniform angular distribution. On the other hand, a shallow ellipse may provide a substantially uniform angular distribution, but with less intensity.
FIG. 2 is included to illustrate the concept of reflector depth. As shown in FIG. 2, elliptical reflector 120 may be thought of as part of an ellipse. The ellipse has two focal points (foci_1 and foci_2). The arc lamp generates light in all directions at the first focal point (foci_1) of the ellipse. The light generated by arc lamp 110 is reflected from elliptical surface 120 over a 0°-360° azimuthal angle (φ) and a 0°-180° polar angle (θ) to the second focal point (foci_2) of the ellipse. The angular distribution of the reflected light depends on the angular magnification provided by the ellipse as the polar angle changes from 0°-180°. The angular magnification depends strongly on the depth of the ellipse, as described in more detail below.
As shown in FIG. 2, the ellipse may be generally described as having a semi-major axis (a), a semi-minor axis (b) and a distance (c) between the first focal point (foci_1) and the mid-point of the ellipse. A deep ellipse is produced when (a˜c)>>b, or when the eccentricity of the ellipse is near 1. For example, the ellipse can be modeled by the equation:a2=b2+c2  EQ. 1where c is defined as the distance between the first focal point (foci_1) and the mid-point of the ellipse. The eccentricity of the ellipse is, therefore, defined as:e=c/a  EQ. 2When (a˜c)>>b (i.e., in a deep ellipse), the distance (c) is approximately equal to the semi-major axis (a). This renders the eccentricity (e) of the deep ellipse substantially equal to 1.
The angular magnification of a deep ellipse varies significantly as the polar angle changes from 0°-180°. This produces a highly uneven angular distribution at the illumination plane 140 of the illumination system. On the other hand, the angular magnification of a shallow ellipse tends to be relatively constant across the polar angles. This produces a relatively uniform angular distribution of reflected light at the illumination plane. However, it is generally desirable to maximize the intensity of light generated by the illumination system to increase the detection resolution and sensitivity of the inspection system. As described in more detail below, many incoherent illumination systems use deep ellipses with relatively small widths to increase the intensity of light generated by the illumination system.
The intensity of light generated by the incoherent illumination system depends on the width (w), as well as the depth (d) of the elliptical reflector. In particular, the peak intensity of light supplied to the homogenizer entrance is a function of the eccentricity (e=c/a), which is close to 1 for a deep ellipse, and the amount of the light collected by the ellipse. The maximum collection angle or polar angle (θ) determines the amount of light that can be collected by the reflector and is directly related to the depth (d) of the elliptical reflector and the width (w) of the reflector opening. In particular, the total amount of light collected by elliptical reflector 120 is determined by the ratio [d−(a−c)]/w. When combined with the eccentricity (e=c/a), the maximum collection angle determines the peak intensity/brightness of the light supplied to the homogenizer entrance. In order to increase resolution and sensitivity, it is important that this peak intensity be conserved at subsequent stages of the illumination system.
Homogenizer 150 is often used to improve the spatial uniformity of the light generated by the illumination system. A homogenizer or lightpipe is generally a solid glass rod or tubular passageway through which light rays can travel by total internal reflection. Homogenizers can have various cross-sectional shapes, such as rectangular (180, FIG. 3A), circular (190, FIG. 3B) or square (not shown). Rectangular and square homogenizers can also be formed by arranging four mirrors, so that the reflective surfaces of the mirrors create a rectangular or square passageway.
Rectangular homogenizers improve the spatial uniformity of the generated light by randomizing the light as it bounces through the homogenizer. Specifically, rectangular homogenizers are used to scramble the spatial distribution of the light, so that the light will be uniformly distributed at the field stop (not shown) of the illumination system. After passing through homogenizer 150, the light is directed by optical lens 160 to the pupil plane 170 of the illumination system. The optical lens 160 is generally located one focal length of the lens away from the exit of the homogenizer 150. The pupil plane 170 is generally located one focal length of the lens away from the optical lens 160.
Although homogenizer 150 improves the spatial uniformity of the light generated at the field stop, the light remains non-uniform with respect to angular distribution at the pupil plane 170 of the illumination system. In other words, rectangular homogenizers preserve the angles at which light bounces off the internal surfaces of the homogenizer. Rectangular homogenizers do not redistribute the angles at which light travels through the homogenizer, and therefore, do not change the angular distribution of the light produced at the pupil plane 170.
In some cases, an illumination system including a plasma arc lamp 110, a deep arc elliptical reflector 120 and a rectangular homogenizer 150 may produce a highly uneven angular distribution, with a majority of the light concentrated in the low angle or low numerical aperture (NA) range. In addition to reducing detection resolution and sensitivity, the highly concentrated low NA light generated by illumination system 100 may create problems for high NA bright-field (BF) and dark-field (DF) inspection systems. In particular, the highly concentrated low NA light may create light budget and lens damaging issues in such systems.
It is, therefore, desirable to provide an improved illumination system, which balances the energy distribution in the pupil plane, as well as the field stop of the illumination system. Preferably, the improved illumination system would be configured to redistribute an angular distribution of the light evenly over an entire numerical aperture (NA) transmission window of the inspection system with minimum light loss. In one embodiment, the improved illumination system may do so by providing a novel homogenizer designed to convert a majority of the low NA light to high NA light (or vice versa) at a specific location along the homogenizer.